Zero Discharge Riserless Drilling – Alternative to Pumping Dumping

摘要

A current industry practice generally referred to asrnPumping & Dumping speaks to a method of establishingrnsubsea locations via riserless drilling. Riserless drilling of toprnholes and often by batch drilling a number of locations withinrna lease block is an attractive practice because a much lessrnexpensive floating rig may be utilized. Without the weight of arnmarine riser system and subsea BOP stack to buoy, smallerrnrigs can be efficiently used to establish subsea locations. Later,rna larger semi submersible or dynamically positioned drill shiprnwith marine riser and subsea (or surface) BOP drills to therntotal depth objective.rnUnder the best of circumstances, seawater is used as therndrilling fluid. Mud and cuttings are discharged at the mudlinernonto the sea floor, thus the term “pumping & dumping”.rnWhere there is a risk of drilling riserless into a relativelyrnshallow-to-the-mudline geohazard of abnormal pressure, arnheavy, viscous mud may be required, this as a means ofrnoverbalancing the zone of abnormal pressure. Another purposernfor overbalance may be to enhance wellbore stability. Shallowrngeohazards may be shallow gas or abnormally pressuredrnaquifers. For example, continuing to drill riserless withrnseawater in the presence of a shallow water flow hazard mayrnresult in loss of the well. A volcanic-like eruption may flowrnfrom the subsea wellhead with such volume and velocity thatrnthe particulate fall-out could cover this and nearby batchdrilledrnlocations.rnA technology that offers the ability to overbalancernabnormally pressured zones when drilling riserless withrnseawater that enables more precise wellbore pressurernmanagement with fewer interruptions to drilling ahead isrnindicated. A technology that recovers perhaps expensive mudrnand avoids the discharge of annulus returns into the sea furtherrndefines a growing industry need for a dual gradient riserlessrndrilling system.rnThis paper discusses several approaches to accomplishingrndual gradient riserless drilling. The drilling medium, whetherrnseawater or a heavy viscous mud, is returned to the rig ratherrnthan single passed onto the seafloor. Such a method may alsornbe referred to as “zero discharge riserless drilling”.